1. Field of the Invention
The present invention relates to an injection molding machine for molding an optical element with a mold. More particularly, it relates to an injection molding machine which controls temperature with high accuracy to improve molding accuracy.
2. Description of the Related Art
There have conventionally been manufactured various molded items by injection molding. For example, JP Unexamined Patent Publication No. 10-323872 discloses an injection molding machine for molding disks utilized as information recording media. FIG. 4 shows schematic structure of the molding injection machine directed to the Publication No. 10-323872. The molding injection machine includes a fixed platen 101 and a cylinder 103. Four tie-bars 104 are laid between the fixed platen 101 and the cylinder 103 to link them. The tie-bars 104 are supporting a movable platen 105 which is slidable. The movable platen 105 is driven by the cylinder 103. A fixed mold 107 and a movable mold 108 are attached to the fixed platen 101 and the movable platen 105, respectively. The movable mold 108 is clamped to the fixed mold 107 by the cylinder 103. With this state, resin material is supplied from an injection unit 111 to mold a disk.
Such an injection unit of an injection molding machine is required to control temperature with high accuracy so as to carry out stable mold injection with high accuracy. If temperature is too low, molding accuracy deteriorates. If temperature is too high, resin deteriorates or gets stringiness. Therefore, there has conventionally been provided heat adjusting means for a nozzle portion and a cylinder portion. For example, JP Unexamined Patent Publication No. 2005-7629 discloses an injection unit equipped with a heater and a temperature sensor at each zone. In the Publication No. 2005-7629, a thermo couple is disclosed as an example of temperature sensor. Since a thermo couple was cheap and satisfied sufficient accuracy in comparison with conventional accuracy demand, it was used widely.
However, with the advance of accuracy of optical elements, temperature control accuracy managed by the conventional injection molding machine has become insufficient. Especially, as to micro molding machine, its one-shot volume is small and its nozzle portion sometimes has volume of several shots. As one-shot volume is smaller, higher accuracy in proportion to the smallness is required for injection performance and temperature control accuracy. In the case the conventional injection molding machine intends to control temperature with high accuracy, it is required to use a temperature sensor of higher accuracy. As an example of a temperature sensor of which accuracy is higher than a thermo couple, a platinum temperature-sensing element, a quartz temperature sensor, and the like can be utilized. However, they are expensive items. As described, the conventional injection molding machine has plural temperature control means at respective zones and if all of them are replaced with highly accurate temperature sensors, the replacement accompanies considerable cost-up.